Distraction instrument for use in anterior cervical fixation surgery

ABSTRACT

An intervertebral distraction tool has a clamshell head with upper and lower halves, each having a curvate outer surface and a flat inner surface. The distal side of the head is hinged so that the head opens and closes from the proximal side of the head. The hinge is a separating hinge that allows the halves to not only angulate with respect to one another about the hinge axis, but also to vertically separate from one another at the hinge. A distraction separator has decreasing taper at its distal end and a longitudinal bore that accommodates the elongate shaft of the trial so that the separator can be moved longitudinally relative to the shaft. Upon forward movement of the separator, the tapered upper and lower surfaces engage the flat inner surfaces of the head, causing the halves to angulate about the hinge axis of the head, thereby opening the head.

FIELD OF THE INVENTION

[0001] This invention relates generally to a medical instrument for usein anterior cervical spine surgery and more specifically to adistraction instrument used in the treatment and correction ofpathologies in the cervical spine.

BACKGROUND OF THE INVENTION

[0002] The spinal column consists of more than 20 discrete bones whichare joined together in a highly complex arrangement which houses andprotects critical elements of the nervous system and serves as astructural framework around which there are innumerable peripheralnerves and circulatory bodies in close proximity. In spite of thesecomplicating features, the spine is a highly flexible structure, capableof a high degree of curvature and twist in nearly every directionpermitting human beings a high degree of physical dexterity. In generalthese bones are coupled sequentially to one another by tri-jointcomplexes which consists of an anterior intervertebral disc and the twoposterior facet joints. The anterior intervertebral discs of adjacentbones are cushioning cartilage spacers.

[0003] The bones and connective tissue of the neck, or cervical spine,are particularly complex in that they are smaller, are the most flexiblein the spinal column, and are adjacent to the most critical circulatory,respiratory, and digestive tissues in the body. Long term degenerationof the cartilage spacers and/or trauma, can cause adjacent bones of thecervical spine to collapse together and/or become axially displaced(that is, become segmentally offset). These and other failure modes cancause significant pain as well as catastrophic neurological problems.

[0004] The traditional course of surgical treatment can include theimplantation of a plate which seats against the anterior surfaces ofadjacent bones, and the fixation of the plate thereto with bone screwsimmobilizes the two bones in the distracted and aligned position.

[0005] Concerns for the overall safety of this method of treatment,however, include the risk of screw pull out and/or breakage, or platebreakage, each of which are accompanied by the severe risk of esophegealperforation, which can be fatal if not treated immediately andaggressively. The surgical effectiveness of this treatment is enhanceddramatically if the adjacent bones are able to fuse together across thedistracted space, thus forming a single bony element which can supportthe neck without needing to load the metal plate (thus risking breakageor other failure mode). The problem is that the distraction increasesthe distance between the fusing bones, and the need to create the bonebridge requires that the bones be close enough to grow together. Thesecompeting factors have made the use of porous material as a spacermaterial desirable. This spacer physically braces the bones at thedesired positions, permitting the bones to fuse across the entireassembly.

[0006] Similar techniques have been employed in other spinalinfirmities, including collapsed disc spaces in the thoraco-lumbarregions of the spine. The present invention is therefore applicable tothe treatment of any spinal disorder in which the space betweenvertebral bones needs to be surgically separated (the bones distracted),and then fused to one another, however, for the purposes of thisinvention disclosure, only the application to the cervical spine shallbe described hereinbelow.

[0007] Before the implant may be inserted into the space, however, theheight of the disc space (the distance between the opposing end platesof the adjacent bones) must be restored. Restoring the appropriateheight and orientation of the vertebral bones and the intervertebralspace is critical, and when done properly (that is, through a series ofsequentially larger trial spacers) is also instructive for the purposesof determining the appropriate size of the implant to be provided.

[0008] It shall be understood that the use of an anterior cervical platein conjunction with the foregoing will be the preferred method ofsurgical treatment, however, its use should not be considered arequirement in the practice of the present invention, which should beviewed as a wholly separate and independent surgical process.

[0009] It is, therefore, an object of the present invention to provide anew and novel treatment for cervical disc pathology, as well as for thetreatment of spinal pathologies in general.

[0010] It is, correspondingly, another object of the present inventionto provide an intervertebral distraction tool that more accurately andeasily separates collapsed intervertebral spaces and restores the properanatomical orientation of cervical bones.

[0011] It is further an object of the present invention to provide animplantable intervertebral spacer device, and insertion instrument,which permits more anatomically appropriate and rapidly osteogenicfusion across the intervertebral space.

[0012] Other objects of the present invention not explicitly stated willbe set forth and will be more clearly understood in conjunction with thedescriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

[0013] The preceding objects of the invention are achieved by thepresent invention, which provides an intervertebral distraction tool andan intervertebral spacer device and insertion instrument.

[0014] A preferred embodiment of an intervertebral distraction tool(also referred to herein as a trial) of the present invention has aproximal end defined by an elongate shaft, and a distal end defined by aclamshell head. The head has upper and lower halves, each having acurvate outer surface and a flat inner surface. The distal side of thehead is hinged so that the head opens and closes from the proximal sideof the head. Preferably, the hinge is a separating hinge that allows thehalves to not only angulate with respect to one another about the hingeaxis, but also to vertically separate from one another at the hinge.Preferably, the proximal side of the head has a tapered lip thatfacilitates insertion of the distal end of the distraction separator(described below) in between the halves. The head further has a pair ofposts and corresponding bores into which the posts compress fit when thehead is closed. The force of the compression fit holds the head closed,so that the head can be held closed during the insertion of the headinto the target intervertebral space. When the distraction separator isactuated to open the head, the compression force is overcome to allowthe head to open.

[0015] A preferred embodiment of a distraction separator of the presentinvention has an elongate shaft having a longitudinal bore. The distalend of the shaft has a decreasing taper defined by upper and lowersurfaces that are angled toward one another, terminating at a distalface of the separator. The longitudinal bore accommodates the elongateshaft of the trial so that the separator can be moved longitudinallyrelative to the shaft. The longitudinal movement is preferably effectedby simply pushing the proximal end of the separator toward the head ofthe trial. Upon forward movement of the separator, the tapered upper andlower surfaces engage the flat inner surfaces of the head, causing thehalves to angulate about the hinge axis of the head, thereby opening thehead. Further advancement of the separator in between the halves causesthe halves to not only angulate with respect to one another about thehinge axis, but also to vertically separate from one another at thehinge, due to the separating hinge. Once the head has been opened asdesired by the surgeon, extraction of the separator from between thehalves and removal of the head from the intervertebral space leaves thedistracted space ready for accepting additional trials or for insertionof a spacer of the present invention.

[0016] A preferred embodiment of a spacer device of the presentinvention has a thickness that is preferably predetermined to be adistance that is to be established between two vertebral discs that areto be fused together. The spacer further has an overall rectangularshape with rounded corners to limit interference with surrounding tissueduring the insertion procedure. The spacer further has upper and lowersurfaces that are curvate to allow them to more easily fit into andremain in the intervertebral space, with each curvate surface seatingwithin the vertebral cavity presented to the surface when the spacer isdisposed in the intervertebral space. The spacer is preferably formedfrom a material that can withstand compressive forces that are presentwithin an intervertebral space, and further is preferably formed of aporous material that facilitates bone growth thereinto for a successfulfusion procedure. Further preferably, the upper and lower surfaces arerough surfaces that will stimulate bone growth into the porous material.The spacer further has a plurality of linear grooves to facilitateinsertion of the spacer into the intervertebral space. Each of thegrooves has a depth, preferably is rounded, and preferably has a smoothsurface.

[0017] A preferred embodiment of a spacer insertion tool of the presentinvention has a scissor-style body having upper and lower arms hingedabout a hinge rod, each of the arms having a distal end having a head,each of the arms having a proximal end having a gripping handle. Each ofthe heads has an inner surface having a pair of linear protrusions thatare spaced and dimensioned to fit within the linear grooves of thespacer when the heads are closed about the spacer. That is, when thearms are actuated by a surgeon gripping and bringing together thehandles, the arms hinge about the hinge rod and therefore the heads arebrought to bear on the spacer such that each of the protrusions fitsinto a respective one of the grooves, and the spacer is held between theheads by the continual compression of the spacer in this manner.Accordingly, the surgeon, while holding the spacer with the arms, caninsert the spacer into the intervertebral space that has beendistracted.

[0018] It should be noted that when the spacer is held with the arms,the upper and lower curvate surfaces are preferably not engaged by theheads. That is, the dimensions of the protrusions and/or the grooves aresuch that spaces are present between the upper and lower curvatesurfaces and the inner surfaces of the heads. This feature is primarilyprovided so that when the spacer is released from the insertion tool,the upper and lower surfaces are not altered during the removal of theheads from the spacer.

[0019] In order to facilitate removal of the heads from the spacer sothat the spacer can be left in the intervertebral space, the insertiontool is configured such that the arms can be moved longitudinallyrelative to a bracing rod. In the illustrated embodiment, this featureis provided inasmuch as the insertion tool is provided with a bore,preferably though the hinge rod, and the bracing rod is disposed throughthe bore.

[0020] In operation to remove the heads from the spacer while holdingthe spacer in the intervertebral space, the distal end of the bracingrod is held against the spacer by the surgeon while the surgeon pullsthe handles. The pulling of the handles while the spacer is held in theintervertebral space causes the protrusions to slide out of the groovesuntil the spacer is released from the heads and is compressed betweenthe vertebral discs by the natural compression force present in thespine. Accordingly, the upper and lower surfaces of the spacer, beingcurvate, easily fit into and remain in the intervertebral space, witheach curvate surface seating within the vertebral cavity presented tothe surface when the spacer is disposed in the intervertebral space.Further, the surfaces being rough, stimulate bone growth, and beingporous, provide for bone growth into the material to facilitate fusion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIGS. 1a-c show an intervertebral distraction tool in two sideviews (FIG. 1a where the trial is closed, and FIG. 1b where the trial isopen) and a top view, respectively.

[0022]FIGS. 2a-c show a distraction separator of the present inventionin side, top and side views, respectively (with FIG. 2c showing theseparator advanced within the trial of FIGS. 1a-c).

[0023]FIGS. 3a-c show a spacer device of the present invention in side,front and top views, respectively.

[0024]FIGS. 4a-b show a spacer insertion tool of the present inventionin a side view and a front view, respectively, each showing theinsertion tool holding the spacer of FIGS. 3a-c of the presentinvention, and the front view omitting the arms and handles of theinsertion tool.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] While the present invention will be described more fullyhereinafter with reference to the accompanying drawings, in whichparticular embodiments and methods of implantation are shown, it is tobe understood at the outset that persons skilled in the art may modifythe invention herein described while achieving the functions and resultsof this invention. Accordingly, the descriptions that follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope. Like numbers refer to similar features oflike elements throughout.

[0026] Referring now to FIGS. 1a-c, an intervertebral distraction tool100 is shown in two side views (FIG. 1a where the trial 100 is closed,and FIG. 1b where the trial 100 is open) and a top view, respectively.The trial 100 has a proximal end 132 and a distal end 134, the proximalend 132 being defined by an elongate shaft 136, and the distal end 134being defined by a clamshell head 138. The head 138 has an overallrectangular shape with rounded corners to limit interference withsurrounding tissue during the distraction procedure. It should beunderstood that the shape of the head 138 can be any shape that fitsbetween vertebral bodies, that other embodiments of the presentinvention may not use rectangular shaped heads. Further, otherembodiments of the present invention may or may not have rounded cornerson the head 138, depending on the clinical necessity or desirability ofsuch or similar features.

[0027] The head 138 has upper 102 and lower 104 halves, each having acurvate outer surface 106, 108 and a flat inner surface 110, 112. Aswill be described below, the curvature of the outer surfaces 106, 108facilitates the distraction of the vertebral bodies, but it should beunderstood that the curvature is not necessary for the functioning ofthe present invention, and that outer surfaces of alternate conformitycan be used without departing from the scope of the present invention.Similarly, the flat contour of the inner surfaces 110, 112 is preferred,but other contours can be provided within the scope of the presentinvention.

[0028] The distal side 140 of the head 138 is hinged so that the head138 opens and closes from the proximal side 142 of the head 138.Preferably, as shown, the hinge 114 is a separating hinge that allowsthe halves 102, 104 to not only angulate with respect to one anotherabout the hinge axis, but also to vertically separate from one anotherat the hinge. This is achieved by the use of a hinge pin 116 having adiameter smaller than the height of the each half's hinge bore 118, 120.It should be understood that the use of a separating hinge is notnecessary for the functioning of the present invention, but rather ispreferred, and that non-separating hinges, and other mechanisms thatallow the halves to angulate with respect to one another (with orwithout separating vertically) can be provided within the scope of thepresent invention.

[0029] Preferably, as shown, the proximal side 142 of the head 138 has atapered lip 122 that facilitates insertion of the distal end of thedistraction separator (described below) in between the halves 102, 104(best shown in FIG. 2c, described below). The head 138 further has apair of posts 124, 126 and corresponding bores 128, 130 into which theposts 124, 126 compress fit when the head 138 is closed. The force ofthe compression fit holds the head 138 closed, so that the head 138 canbe held closed during the insertion of the head 138 into the targetintervertebral space (the space into which the spacer of the presentinvention, described below, will be placed to facilitate fusion of theadjacent vertebral bodies). When the distraction separator (describedbelow) is actuated to open the head 138, the compression force isovercome to allow the head 138 to open. It should be understood that thepost and bore configuration is preferable, but that other mechanisms canbe provided to hold the head 138 closed, without departing from thescope of the present invention.

[0030] Referring now to FIGS. 2a-c, a distraction separator 200 of thepresent invention is shown in side, top and side views, respectively(with FIG. 2c showing the separator 200 advanced within the trial 100described above). The separator 200 has an elongate shaft 202 having alongitudinal bore 208 and a proximal end 204 and a distal end 206. Thedistal end 206 has a decreasing taper defined by upper 210 and lower 212surfaces that are angled toward one another, terminating at a distalface 214 of the separator 200. The longitudinal bore 208 accommodatesthe elongate shaft 136 of the trial 100 so that the separator 200 can bemoved longitudinally relative to the shaft 136. The longitudinalmovement is preferably effected by simply pushing the proximal end 204toward the head 138. However, it should be noted that other mechanismsfor moving the separator 200 can be used, including, but not limited to,for example, providing threads on the walls of the longitudinal bore208, corresponding threads on the outer surface of the shaft 136, andproviding for rotational movement of the threaded portions of the shaft136 relative to the distal end 134 of the shaft 136, so that rotation ofthe shaft 136 causes forward movement of the separator 200 relative tothe shaft 136 and reverse rotation of the shaft 136 causes backwardmovement of the separator 200 relative to the shaft 136.

[0031] As best shown in FIG. 2c, upon forward movement of the separator200, the tapered upper and lower surfaces 210, 212 engage the flat innersurfaces 110, 112 of the head 138, causing the halves 102, 104 toangulate about the hinge axis of the head 138, thereby opening the head138. The further the separator 200 is advanced in between the halves102, 104, the further the head 138 opens. Therefore, the surgeon canvary the amount of distraction. In addition, further advancement of theseparator 200 in between the halves 102, 104 causes the halves 102, 104to not only angulate with respect to one another about the hinge axis,but also to vertically separate from one another at the hinge, due tothe separating hinge 114. Once the head 138 has been opened as desiredby the surgeon, extraction of the separator 200 from between the halves102, 104 (preferably by simply pulling on the proximal end 204, butalternatively by other mechanisms, as described above) and removal ofthe head 138 from the intervertebral space leaves the distracted spaceready for accepting additional trials (for example, thicker trialsand/or trials that open to greater distances) or for insertion of aspacer of the present invention (described below). With regard toaccepting additional trials, it should be noted that a plurality oftrials can be provided, each having an individually unique thicknessand/or an individually unique distance to which the head of the trialcan be opened. Therefore, the surgeon can distract the space to anydesired distance through the use of one or more of such trials.

[0032] With regard to inserting spacers, and referring now to FIGS.3a-c, a spacer 300 of the present invention is shown in side, front andtop views, respectively. The spacer 300 has a thickness 302 that ispreferably predetermined to be a distance that is to be establishedbetween two vertebral discs that are to be fused together. The spacer300 further has an overall rectangular shape with rounded corners andedges to limit interference with surrounding tissue during the insertionprocedure. It should be understood that the shape of the spacer 300 canbe any shape that fits between vertebral bodies, that other embodimentsof the present invention may not use rectangular shaped heads. Further,other embodiments of the present invention may or may not have roundedcorners or edges on the spacer 300, depending on the clinical necessityor desirability of such or similar features.

[0033] The spacer 300 further has upper and lower surfaces 304, 306 thatare curvate to allow them to more easily fit into and remain in theintervertebral space, with each curvate surface 304, 306 seating withinthe vertebral cavity presented to the surface when the spacer 300 isdisposed in the intervertebral space. These surfaces give the spacer 300a shape similar to a pillow. It should be understood that the curvatureis preferable but not necessary for the functioning of the presentinvention, and that surfaces of alternate conformity can be used withoutdeparting from the scope of the present invention.

[0034] The spacer 300 is preferably formed from a material that canwithstand compressive forces that are present within an intervertebralspace, and further is preferably formed of a porous material thatfacilitates bone growth thereinto for a successful fusion procedure.Examples of materials suitable for this purpose include porous metalsand compacted wire meshes, bone morphogenic protein, and polylacticlactic acid. Further preferably, the upper and lower surfaces 304, 306are rough surfaces that will stimulate bone growth into the porousmaterial.

[0035] The spacer 300 further has a plurality of linear grooves 308,309, 310, 311 to facilitate insertion of the spacer 300 into theintervertebral space (as described below). For reasons explained ingreater detail below, each of the grooves has a depth 312, preferably isrounded, and preferably has a smooth surface. It should be noted thatalthough two grooves are shown on each upper and lower surface of thespacer, more or fewer grooves can be used without departing from thescope of the present invention.

[0036] It should be understood that a plurality of spacers 300 of thepresent invention can be provided, having individually uniquethicknesses. Preferably, the thicknesses are predetermined to bedistances that are to be established between two vertebral discs thatare to be fused together. That is, depending on the clinicalapplication, the intervertebral space may be distracted to a particulardistance (e.g., by the distraction process described above), and aplurality of spacers can be provided with different thicknesses so thatat least one has a thickness matching the particular intervertebraldistance in any given clinical application. Preferably, each of theplurality of spacers has linear grooves that preferably are of the samedepth 312 from spacer to spacer (so that if desired a single insertiontool (e.g., of a type described below) can be used to insert more thanone of the spacers), preferably are rounded, and preferably have smoothsurfaces.

[0037] Referring now to FIGS. 4a-b, a spacer insertion tool 400 of thepresent invention is shown in a side view and a front view,respectively, each showing the insertion tool 400 holding the spacer 300of the present invention, and the front view omitting the arms andhandles of the insertion tool. The insertion tool 400 has ascissor-style body 402 having upper 404 and lower 406 arms hinged abouta hinge rod 408, each of the arms 404, 406 having a distal end 410, 412having a head 414, 416, each of the arms 404, 406 having a proximal end418, 420 having a gripping handle 422,424. It should be noted that whilethe insertion tool 400 of the illustrated embodiment has a scissor-stylebody 402 to open and close the heads 414, 416, other embodiments of thepresent invention may have bodies of other styles that may or may notutilize hinges or similar structures to open and close the heads 414,416.

[0038] Each of the heads 414, 416 has an inner surface 426, 428 having apair of linear protrusions 430, 432, 434, 436 that are spaced anddimensioned to fit within the linear grooves 308, 309, 310, 311 of thespacer 300 when the heads 414, 416 are closed about the spacer 300. (Asnoted above, although two grooves are shown on each upper and lowersurface of the spacer, more or fewer grooves can be used withoutdeparting from the scope of the present invention, and hence more orfewer corresponding protrusions can be used on the heads 414, 416without departing from the scope of the present invention; it shouldalso be noted that it is not necessary for the number of grooves toequal the number of protrusions.) That is, when the arms 404, 406 areactuated by a surgeon gripping and bringing together the handles 422,424, the arms 404, 406 hinge about the hinge rod 408 and therefore theheads 414, 416 are brought to bear on the spacer 300 such that each ofthe protrusions 430, 432, 434, 436 fits into a respective one of thegrooves 308, 309, 310, 311, and the spacer 300 is held between the heads414, 416 by the continual compression of the spacer 300 in this manner.Accordingly, the surgeon, while holding the spacer 300 with the arms404, 406, can insert the spacer 300 into the intervertebral space thathas been distracted (e.g., according to the procedures described above).

[0039] It should be noted that when the spacer 300 is held with the arms404, 406, the upper and lower curvate surfaces 304, 306 are preferablynot engaged by the heads 414, 416. That is, the dimensions of theprotrusions 430, 432, 434, 436 and/or the grooves 308, 309, 310, 311 aresuch that spaces 438, 440 are present between the upper and lowercurvate surfaces 304, 306 and the inner surfaces 426, 428 of the heads414, 416. This feature is primarily provided so that when the spacer 300is released from the insertion tool 400 (as described below), the upperand lower surfaces 304, 306 (preferably being rough and porous as notedabove) are not altered during the removal of the heads 414, 416 from thespacer 300. During the removal, the heads 414, 416 only engage thespacer 300 at the grooves 308, 309, 310, 311, and therefore the upperand lower surfaces 304, 306 are not at risk for being damaged.

[0040] In order to facilitate removal of the heads 414, 416 from thespacer 300 so that the spacer 300 can be left in the intervertebralspace, the insertion tool 400 is configured such that the arms 404, 406can be moved longitudinally relative to a bracing rod 444. In theillustrated embodiment, this feature is provided inasmuch as theinsertion tool is provided with a bore 442, preferably though the hingerod 408 as shown, and the bracing rod 444 that is disposed through thebore 442. It should be understood that other embodiments may use otherconfigurations to effect the same or similar functionality.

[0041] The longitudinal movement of the arms 404, 406 relative to thebracing rod 444 is preferably effected by holding the bracing rod 444and pulling the handles 422, 424. The bracing rod 444 has a proximal end446 and a distal end 448. In operation to remove the heads 414, 416 fromthe spacer 300 while holding the spacer 300 in the intervertebral space,the distal end 448 of the bracing rod 444 is held against the spacer 300by the surgeon (e.g., by gripping and maintaining the position of theproximal end) while the surgeon pulls the handles 422, 424. The pullingof the handles 422, 424 while the spacer 300 is held in theintervertebral space causes the protrusions 430, 432, 424, 436 to slideout of the grooves 308, 309, 310, 311 (the grooves and the protrusionsare preferably rounded and smoothed as noted above to facilitate thissliding) until the spacer 300 is released from the heads 414, 416 and iscompressed between the vertebral discs by the natural compression forcepresent in the spine. Accordingly, the upper and lower surfaces 304, 306of the spacer 300, being curvate, easily fit into and remain in theintervertebral space, with each curvate surface 304, 306 seating withinthe vertebral cavity presented to the surface when the spacer 300 isdisposed in the intervertebral space. Further, the surfaces 304, 306being rough, stimulate bone growth, and being porous, provide for bonegrowth into the material to facilitate fusion.

[0042] It should be noted that other mechanisms for moving the arms 404,406 relative to the bracing rod 444 can be used, including, but notlimited to, for example, providing threads on the walls of the bore 442,corresponding threads on the outer surface of the bracing rod 444, andproviding for rotational movement of the threaded portion of the bracingrod 444 relative to the distal end 448 of the bracing rod 444, so thatrotation of the bracing rod 444 causes forward movement of the arms 404,406 relative to the bracing rod 444 and reverse rotation of the bracingrod 444 causes backward movement of the arms 404, 406 relative to thebracing rod 444.

[0043] It should also be noted that if desired, a plurality of insertiontools 400 can be provided for inserting a plurality of spacers 300,where the spacers 300 are of different thicknesses and the insertiontools 400 open to different distances, so that one or more insertiontools 400 can be used to more effectively grip a particular spacer orset of spacers 300.

[0044] While there has been described and illustrated specificembodiments of the present invention, it will be apparent to thoseskilled in the art that variations and modifications are possiblewithout deviating from the broad spirit and principle of the presentinvention. The invention, therefore, shall not be limited to thespecific embodiments discussed herein.

We claim:
 1. An intervertebral distraction tool, comprising: alongitudinal shaft having a distal end and a proximal end; a trial headat the distal end of the longitudinal shaft, the trial head having aproximal portion, the trial head being laterally separable from thelongitudinal shaft; and a head separator having an engagement portion,the engagement portion of the head separator being longitudinallymovable relative to the longitudinal shaft and engagable by thelongitudinal movement with the proximal portion of the trial head;wherein when the engagement portion of the head separator engages theproximal portion of the trial head, the trial head laterally separatesfrom the longitudinal shaft.
 2. The intervertebral distraction tool ofclaim 1, wherein the trial head comprises first and second jaw members,each having a distal end and a proximal end, the proximal ends being theproximal portion of the trial head, the distal ends of the first andsecond jaw members being hinged to one another at the distal end of thelongitudinal shaft so that the proximal ends of the first and second jawmembers are openable and closeable about the longitudinal shaft.
 3. Theintervertebral distraction tool of claim 2, wherein the each of thefirst and second jaw members has a curvate outer surface and a flatinner surface.
 4. The intervertebral distraction tool of claim 2,wherein the first and second jaw members are hinged by a hinge thatallows the first and second jaw members to angulate with respect to oneanother at the hinge and that allows the distal ends of the first andsecond jaw members to separate from one another at the hinge.
 5. Theintervertebral distraction tool of claim 2, wherein the proximal ends ofthe first and second jaw members are laterally inwardly tapered towardthe distal end of the longitudinal shaft and the engagement portion ofthe head separator is laterally inwardly tapered toward the distal endof the longitudinal shaft, so that when the engagement portion engagesthe proximal ends of the first and second jaw members, the outwardlytapered portion of the engagement portion pushes the inwardly taperedportions of the first and second jaw members laterally away from thelongitudinal shaft.
 6. The intervertebral distraction tool of claim 5,wherein the head separator comprises a longitudinal sleeve having alongitudinal bore through which the longitudinal shaft is disposed sothat the longitudinal sleeve is longitudinally translatable on thelongitudinal shaft, the longitudinal sleeve having a distal end thatincludes the engagement portion.
 7. The intervertebral distraction toolof claim 1, wherein the head separator comprises a longitudinal sleevehaving a longitudinal bore through which the longitudinal shaft isdisposed so that the longitudinal sleeve is longitudinally translatableon the longitudinal shaft, the longitudinal sleeve having a distal endthat includes the engagement portion.
 8. The intervertebral distractiontool of claim 7, wherein the engagement portion of the head separator islaterally inwardly tapered toward the distal end of the longitudinalshaft and the proximal portion of the trial head is laterally inwardlytapered toward the distal end of the longitudinal shaft, so that whenthe engagement portion engages the proximal portion, the laterallyinwardly tapered portion of the engagement portion pushes the laterallyinwardly tapered portion of the trial head laterally away from thelongitudinal shaft.
 9. The intervertebral distraction tool of claim 8,wherein the trial head comprises first and second jaw members, each ofthe first and second jaw members having a proximal end, the proximalends being the proximal portion of the trial head, the first and secondjaw members being separable at the proximal ends, each of the proximalends being laterally inwardly tapered toward the distal end of thelongitudinal shaft, so that when the engagement portion of the headseparator engages the proximal ends of the first and second jaw members,the inwardly tapered portion of the engagement portion pushes theinwardly tapered portions of the first and second jaw members laterallyaway from the longitudinal shaft.
 10. The intervertebral distractiontool of claim 9, wherein each of the first and second jaw members has adistal end, the distal ends of the first and second jaw members beinghinged to one another at the distal end of the longitudinal shaft sothat the proximal ends of the first and second jaw members are openableand closeable about the longitudinal shaft.
 11. The intervertebraldistraction tool of claim 10, wherein the first and second jaw membersare hinged by a hinge that allows the first and second jaw members toangulate with respect to one another at the hinge and that allows thedistal ends of the first and second jaw members to separate from oneanother at the hinge.
 12. An intervertebral distraction tool,comprising: a longitudinal shaft having a distal end; a clamshell-stylehead mounted about the longitudinal shaft at the distal end of thelongitudinal shaft, the head having a hinged end at the distal end ofthe longitudinal shaft and an openable end opposite the hinged end; anda longitudinal sleeve that is longitudinally translatable along thelongitudinal shaft, the longitudinal sleeve having a distal endengageable by the longitudinal translation with the openable end of thehead; wherein when the distal end of the longitudinal sleeve engages theopenable end of the head, the openable end opens.
 13. The intervertebraldistraction tool of claim 12, wherein the openable end has distallytapering surfaces adjacent the longitudinal shaft and the distal end ofthe longitudinal sleeve has a distally tapering ram, such that when theram engages the distally tapering surfaces, the surfaces are separatedfrom the longitudinal shaft.
 14. The intervertebral distraction tool ofclaim 12, wherein the hinged end is hinged by a separating hinge. 15.The intervertebral distraction tool of claim 12, wherein the head hascurvate outer surfaces and flat inner surfaces.